The present exemplary embodiments relate to phosphor blends for the conversion of radiation emitted by a light source. They find particular application in conjunction with converting LED-generated ultraviolet (UV), violet or blue radiation into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation from UV, violet and/or blue lasers as well as other light sources to white light.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where colored light is desired. The color of light produced by an LED is dependent on the type of semiconductor material used in its manufacture.
Colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs), have been produced from Group III-V alloys such as gallium nitride (GaN). To form the LEDs, layers of the alloys are typically deposited epitaxially on a substrate, such as silicon carbide or sapphire, and may be doped with a variety of n and p type dopants to improve properties, such as light emission efficiency. With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LED.
Recently, techniques have been developed for converting the light emitted from LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
By interposing a phosphor excited by the radiation generated by the LED, light of a different wavelength, e.g., in the visible range of the spectrum, may be generated. Colored LEDs are often used in toys, indicator lights and other devices. Manufacturers are continuously looking for new colored phosphors for use in such LEDs to produce custom colors and higher luminosity.
In addition to colored LEDs, a combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue emitting GalnN chips. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complementary color, e.g. a yellow-green emission. The total of the light from the phosphor and the LED chip provides a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
The CRI is commonly defined as a mean value for 8 standard color samples (R1-8), usually referred to as the General Color Rendering Index and abbreviated as Ra, although 14 standard color samples are specified internationally and one can calculate a broader CRI (R1-14) as their mean value. In particular, the R9 value, measuring the color rendering for the strong red, is very important for a range of applications, especially of medical nature.
One known white light emitting device comprises a blue light-emitting LED having a peak emission wavelength in the blue range (from about 440 nm to about 480 nm) combined with a phosphor, such as cerium doped yttrium aluminum garnet Y3Al5O12:Ce3+(“YAG”). The phosphor absorbs a portion of the radiation emitted from the LED and converts the absorbed radiation to a yellow-green light. The remainder of the blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. A viewer perceives the mixture of blue and yellow light as a white light.
The blue LED-YAG phosphor device described above typically produces a white light with a general color rendering index (Ra) of from about 70-82 with a tunable color temperature range of from about 4500K to 8000K. Recent commercially available LEDs using a blend of YAG phosphor and a red phosphor (CaS:Eu2+) provide color temperatures below 4500K with a fixed value of Ra around 90. While such LEDs are suitable for some applications, many users desire a light source with an even higher Ra, one similar to that of incandescent lamps with a value of 95-100.
There are also white LEDs that utilize a UV emitting chip and a phosphor blend including red, green and blue emitting phosphors designed to convert the UV radiation to visible light. One problem with these LEDs is that their spectra generally have “peaks” and “valleys” when compared to CIE reference illuminants of the same CCT, which make them deficient in certain regions of the visible spectrum, especially between 450 and 650 nm. For color critical applications (medical, military, etc.), this is not acceptable.
The International Commission on Illumination (CIE) has developed a system of colorimetric illuminants, each of which is defined by a unique spectral power distribution (SPD). Ref. CIE Standard; CIE Colorimetric illuminants, ISO/CIE 10526-1991 (CIE S001-1986).
Other CIE documents define the choice of reference illuminants, e.g. in CRI calculations, as given in: CIE Method of Measuring and Specifying Colour Rendering Properties of Light Sources, CIE 13.3 (1995). For the purposes of this application, any further mention of a “reference illuminant” has the same meaning as in the aforementioned CIE 13.3 publication, unless specified otherwise. Thus the reference illuminants are theoretical light sources used for comparison and evaluation of real (also referred to as test) light sources of the same CCT. For CCT values less than 5000K, the SPD of the CIE reference illuminant matches that of the black body (a.k.a. Planckian) radiator. For CCT values equal to or greater than 5000K, the reference illuminant represents an idealized phase of daylight.
Whenever numbers are used in addition to the letter to specify an illuminant, they stand for the first 2 digits of the CCT. For example, D65 denotes the most widely used daylight illuminant with a CCT of 6500K. Other important daylight illuminants are D50 and D75, with CCT values of 5000K and 7500K, correspondingly.
One special CIE illuminant is illuminant E (a.k.a. Equal Energy Illuminant), the SPD of which as a function of wavelength is a horizontal straight line. Illuminant E has a fixed CCT of approximately 5450K, and has the property of x=y=z=⅓ in the 1931 CIE chromaticity diagram. Although simulation of this illuminant would be extremely difficult for any existing prior art solution, a simulator source would be very useful e.g. in building spectrometers, calorimeters and other similar devices.
On the other hand, certain other CIE illuminants (A, B, C and F) are generally simulated in a straightforward manner, by using either incandescent (A), filtered incandescent (B and C) or fluorescent (F) lamp sources.
As mentioned earlier, at low CCT values (e.g. less than 3200K), the SPD of the CIE reference illuminant is that of the Planckian radiator, and is easily simulated by incandescent light sources. It becomes however progressively more difficult to simulate the SPD of reference illuminants with real light sources at elevated CCTs—e.g. greater than 3200K, and especially equal to or greater than 5000K, when a switch is made from a Planckian spectrum to a phase of daylight in the reference illuminant. Existing simulators of such illuminants work by altering the spectra of incandescent or halogen lamps though filtration, thereby further decreasing their already low efficiency. This necessitates the use of high power light sources and heat resistant filters in order to achieve acceptable light levels.
Simulators of illuminants approximate the SPD of the illuminant at least within a certain wavelength interval e.g. at least from 450 to 650 nm (where the human eye sensitivity curve is at approximately 10% or higher level of its maximum value) more preferably at least from 400 to 700 nm (where the human eye sensitivity curve is at approximately 1% or higher level of its maximum value), and to within certain tolerance limits, e.g. +/−20%, +/−15% or +/−10%. The broader the wavelength interval and the lower the tolerance limit, the better the simulation of the reference illuminant by the actual source under consideration.
An alternative (and inherently more energy efficient) solution for light sources approximating CIE illuminants makes use of LED chips with substantially different peak wavelengths spanning the range from at least 450 to 650 nm. Although technically feasible, this is an extremely complex and expensive solution, due to the need to maintain dynamic power control over multiple wavelength channels in order to achieve and maintain the requisite SPD simulating that of a reference illuminant. In addition, mixing the light from chips with substantially different peak wavelengths is always a challenge and good color uniformity is difficult to achieve.
It would therefore be desirable to develop new and efficient light sources with a generally flat emission spectrum and conforming to within certain tolerance limits (e.g. +/−20% or better) with CIE reference illuminants e.g. of type D or with illuminant E, at least in the spectral region from 450 to 650 nm, preferably 400 to 700 nm, by using LED chips with substantially the same peak wavelength. The present invention provides new and improved phosphor blends and method of formation, which overcome the above-referenced problems and others.